Do not call fields_for from form_for, to avoid instantiating two builders

1 files changed, 13 insertions, 1 deletions

diff --git a/actionpack/test/template/form_helper_test.rb b/actionpack/test/template/form_helper_test.rb
index c730e3ab74..5e47e4db23 100644
--- a/actionpack/test/template/form_helper_test.rb
+++ b/actionpack/test/template/form_helper_test.rb
@@ -1171,7 +1171,6 @@ class FormHelperTest < ActionView::TestCase
     assert_dom_equal expected, output_buffer
   end
 
-
   def test_form_for_with_format
     form_for(@post, :format => :json, :html => { :id => "edit_post_123", :class => "edit_post" }) do |f|
       concat f.label(:title)
@@ -2693,6 +2692,19 @@ class FormHelperTest < ActionView::TestCase
     end
   end
 
+  def test_form_for_only_instantiates_builder_once
+    initialization_count = 0
+    builder_class = Class.new(ActionView::Helpers::FormBuilder) do
+      define_method :initialize do |*args|
+        super(*args)
+        initialization_count += 1
+      end
+    end
+
+    form_for(@post, builder: builder_class) { }
+    assert_equal 1, initialization_count, 'form builder instantiated more than once'
+  end
+
   protected
 
   def hidden_fields(method = nil)